An external defibrillator is described which maintains the durations of phases of a multiphasic shock waveform within or below desired limits. As the duration of a waveform increases for patients of increased patient impedance, the durations of the phases of a multiphasic shock waveform also increase. Before a maximum duration limit is exceeded, the defibrillator adds another phase to the multiphasic waveform which brings the durations of the phases within the desired range or below a maximum duration limit. Both the number of shock phases and the individual phase durations can be controlled in response to measured patient impedance.